Photovoltaic power plants which capture energy from light, in particular light from the sun, are becoming increasingly used as a means of obtaining energy from renewable resources. Such a plant may have a number of photovoltaic devices designed to capture energy from sunlight and convert it into electrical energy. Such devices typically comprise one or more photovoltaic modules connected in series as a ‘string’ or connected in parallel. Again typically, such modules supply a DC current to some form of converter which converts the electrical energy in the DC current into an AC current suitable for supplying directly to an electrical consumer, or an electrical supply grid which supplies remote consumers. The AC outputs of such converters can in turn be connected in parallel so as to increase the energy supplied to a single AC output.
Some types of photovoltaic device, notably thin-film modules incorporating a TCO (transparent conductive oxide) layer, are prone to irreparable damage, and consequent substantial power losses, resulting from the reaction of glass-sodium with moisture. To avoid accelerated degradation of such photovoltaic devices, it is normally an advantage to control the voltage of the devices with respect to ground in a manner which avoids any active part of the photovoltaic devices having a negative voltage with respect to ground. It is known in the art to accomplish this by use of a voltage source which raises the voltage of the negative terminal of the photovoltaic device to a voltage above that of ground. Alternatively, it may be sufficient to maintain the voltage of the negative terminal at or close to the potential of ground.
With some other types of photovoltaic device, notably those where the terminals are all located on one side of the device—known as ‘back-contact modules’—a reduction of device efficiency has been observed during operation. This appears to be due to a build up of static charge on the surface of the cells which make up the device and can be counteracted by maintaining the cells at a voltage below that of ground. Thus it is normally an advantage to control the voltage of the devices with respect to ground in a manner which avoids any active part of the photovoltaic devices having a positive voltage with respect to ground. It is known in the art to accomplish this by use of a voltage source which lowers the voltage of the positive terminal of the photovoltaic device to a voltage below that of ground. Alternatively, it may be sufficient to maintain the voltage of the positive terminal at or close to that of the ground.
Recently, it has been discovered that under some circumstances back-contacted photovoltaic cells can even benefit from being run (partially) above ground potential. For such “special” back-contacted photovoltaic cells environments, the above given description with respect to thin-film photovoltaic cells is applicable in analogy. The converter in a photovoltaic power plant may take the form of an inverter.
It is known in the art that the means of maintaining one or more terminals at voltages with respect to ground is the use of a voltage source, for example a voltage source which is connected via one connection to ground and maintains a positive or negative voltage as required at its second connection. It is also known that such a voltage source can be connected either directly to one or more terminals of the photovoltaic devices, or to the AC output of one or more of the inverters.
Grounding of the photovoltaic devices or other parts of the photovoltaic power plant may be undertaken in order to comply with local regulations, to facilitate the detection of insulation faults and/or to avoid corrosion and/or avoid the efficiency reduction of the photovoltaic modules.
Detection of insulation faults may be difficult in larger systems due to the rather high leakage currents from the photovoltaic devices, especially in wet conditions. By making a connection to ground and monitoring the currents that flow in such a system, leakage currents can be monitored.
In larger photovoltaic power plants comprising several photovoltaic devices and several inverters, grounding of more than one photovoltaic device may cause currents to run through the ground (ground loops). Ground loops may cause problems with controlling the power plant, increase the risk and/or the rate of corrosion and also increase problems relating to electromagnetic interference (EMI). In order to avoid ground loops, transformer-based inverters can be used, so that the DC and the AC sides of the inverters are separated galvanically. Such inverters are, however, relatively heavy and expensive, and there is a demand for photovoltaic power plants, which may utilise transformer-less inverters and still ensure defined voltages with respect to ground at the photovoltaic devices.
In some prior art photovoltaic power plants the connecting to ground of either the positive or negative terminal of the photovoltaic devices is made in a permanent manner by use of a jumper lead. A disadvantage of this type of power plant is the requirement that a ground connection needs to be physically connected to the positive terminal or the negative terminal of the photovoltaic device. This requires additional hardware and labour to attach. In addition, changing the type of photovoltaic devices at a later date may involve the physical disconnection and/or reconnection of a ground connection, a procedure which is labour intensive and therefore a disadvantage.
In a photovoltaic power plant comprising two or more transformerless inverters, and where the AC outputs of the inverters are connected together, but in which no control of the voltages of the photovoltaic device terminals is undertaken, the quiescent voltage to ground of the terminals may float as is shown in FIG. 1. Here the voltages of three photovoltaic devices are shown, the positive terminal voltages 1a, 1b, 1c and the negative terminal voltages 2a, 2b, 2c. The voltages generated across each device are shown as the difference between these voltages, 3a, 3b and 3c. In this particular embodiment, the positive terminals ‘float’ at roughly the same voltage, V1. This is a function of the topologies of the inverters in use. In other words, some types of transformerless inverters set their positive input (and presumably output) terminals to a common potential.
The voltages generated across each device are not the same. This could be due to a number of reasons. For example, the illumination of the individual photovoltaic devices might be different due to different orientations, shadowing, dirt accumulation etc. The load that the device feeds into the input of the inverter might be different due to the characteristics of individual inverters, or the particular state of the maximum power point tracking system in the inverter. All effects combined can lead to an unpredictability of the generated voltages.
FIG. 2 illustrates a prior art system where the AC outputs of the connected transformerless inverters are maintained at a fixed voltage. In this case the effect on the voltage of the terminals of the photovoltaic devices is to lower them all with respect to ground as can be seen by comparing FIGS. 1 and 2. The voltage bias required to do this is a function of the voltages 3a, 3b, 3c (retrospectively of the negative terminal voltages 2a, 2b and 2c), but since these may vary in an unpredictable way it is required to set the voltage of the bias device connected to the AC outputs to a voltage that ensures that the all of the negative terminals are maintained at a positive voltage. This system of control may be suitable for use of thin-film photovoltaic devices as described above. In FIG. 2 we can see that the application of the voltage bias at the AC outputs has reduced the voltages, but they are all still positive with respect to ground.
However, such a system utilising a fixed voltage bias may mean that the voltage bias is maintained at a needlessly large amount, since it will need to be of a size capable of maintaining the highest voltage produced by any of the connected photovoltaic devices at above ground at any time of day or, say, of the year, and for any state of the devices.
Therefore, there is a need for an improved way of fixing the potential of a photovoltaic power plant with respect to ground.